megalith vs dolmen

megalith and dolmen both are nouns.

Word	Noun	Adjective	Verb	Adverb
megalith	Yes	No	No	No
dolmen	Yes	No	No	No

As nouns, dolmen is a hyponym of megalith; that is, dolmen is a word with a more specific, narrower meaning than megalith:

megalith: memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)
dolmen: a prehistoric megalithic tomb typically having two large upright stones and a capstone

Other hyponyms of megalith include cromlech, portal tomb, menhir, standing stone.

megalith (noun)	dolmen (noun)
memorial consisting of a very large stone forming part of a prehistoric structure (especially in western Europe)	a prehistoric megalithic tomb typically having two large upright stones and a capstone